Related apparatuses are known. Thus EP 228633 B1 to Aeromatic AG discloses an apparatus termed Roto-Processor.TM. which has two treatment zones, an inner zone surrounding the rotating disc and an outer fluidized bed zone. Such an apparatus is used for several purposes, for example wet granulation pellettizing where a starting powder is granulated with water or binder solution and, if reuired, subsequently dried in the fluid bed zone, or for coating by spraying a liquid onto core material where several layers of coating can be applied using either the same or different coating liquids. The apparatus can also be used for layering from liquids where a starting product, e.g. inert cores, crystals etc., is being sprayed with an active substance dissolved or suspended in a liquid which liquid is simultaneously dried. Pellets can also be formed by layering from powders where a substance is added as a powder and is bound to the surface of cores or pellets by simultaneously spraying a liquid binder and subsequently or simultaneously drying the resulting pellets.
Another apparatus and a method for producing pellets by layering powder onto particles are disclosed in EP 505319 B1, where liquid and powder are introduced into the particle layer on the rotating disc by means of a nozzle protruding slightly into the zone above the disc.
A similar apparatus is disclosed in EP 526394 A2, which is specially designed for melt coating of particles.
Various auxiliary devices for these types of apparatuses are known. According to U.S. Pat. No. 4,556,175 the apparatus is supplemented with an agitator and in U.S. Pat. No. 4,623,098 furthermore a disintegrator is included.
EP-608844 A1 also deals with a method and an apparatus of the type in question for powder layering and is concerned with the adjustment of the relative amounts of liquid and solids in the process. According to the specification baffle means can be provided to the inner wall of the granulating vessel or the rotary disc for helping tumbling and fluidizing of the material being treated. The specification also mentions the possibility of introducing gas for drying through a portion of the side wall of the granulating vessel.
In the apparatuses in question the material to be treated is kept moving by a combination of mechanical force and gas flow. The material is propelled by the friction which exists between the rotating disc and the material. When the material gains velocity by the shear force in the disc's rotary direction, it will be moved in the direction of the chamber wall by the centrifugal force.
The processes: Granulation, powder layering and coating all demand wetting of the material, and it is essential for the achievement of a uniform product having the desired characteristics as to particle size, particle size distribution, physical strength, solubility etc., that the wetting process is performed very evenly on all particles forming the material. The wetting is usually made by atomizing a liquid by means of a two-fluid nozzle using a pressurized gas, e.g. pressurized air. It is, therefore, important that the material can be moved around inside the chamber in a controlled manner so that each part of the material passes the spray nozzle substantially the same number of times.
This uniform wetting of the particles is even more important in apparatuses of the type in question than in the high-shear apparatuses where the intensive mechanical processing facilitates a rapid equalizing of humidity on the particle surfaces.
With a view of obtaining a controlled movement of the particulate material and also a high drying capacity apparatuses resembling the one forming basis for the present invention have been provided with means for keeping the particulate material in fluidized condition.
An example of such an apparatus is disclosed in DE 37 05 343 C2. In said patent specification fluidizing gas is introduced through a number of slots in a rotating member in the bottom of the apparatus and also through the walls in the upper part thereof. The gas introduced through the walls form an angle to the radial direction, thereby increasing the velocity of the vortex-like movement in the chamber of the particulate material being treated.
However, for certain applications a fluidization of the material being treated is inappropriate in view of the result desired. Especially in powder layering processes the fluidizing gas blows off the finest particles and the contact between fine particles and cores, unto which the fine particles shall adhere, becomes insufficient.
It is, therefore, an object of the present invention to provide an apparatus of the discussed type, in which a controlled and uniform movement of the particulate material is obtained without fluidizing said material to any substantial extent. By avoiding fluidization a more efficient compaction is obtained, which is important especially in powder layering processes.
Besides, the consumption of pressurized gas is substantially reduced in comparison to the consumption when the particulate material has to be fluidized.
Most of the processing apparatuses of the type in question, that means apparatuses having a rotary disc forming the bottom of a chamber and having means for providing a gas stream upward through a slot between the disc and the chamber wall, hitherto commercially available are usually dimensioned to produce batches of product granulae being typically less than 20 kg and typical batch sizes are 10-15 kg.
There is a need for that type of apparatuses having a substantial higher production capacity. However, it has been experienced that simple upscaling of the dimensions of the known apparatuses to enable treatment of larger batches results in inferior and uneven product quality. Especially the size distribution of the granulae becomes too wide, and in layering or coating operations the variation between the treatment received by the individual particles becomes unacceptable high. This results in lacking reproducibility which is a serious drawback, especially when pharmaceutical products are processed.
This is due to the fact that while the momentum transferred from the disc to the particulate material, possibly increased by crevices or bulges on the disc, and the influence of the upward flow of gas between the periphery of the disc and the chamber wall is sufficient to obtain the desired movement of the particulate material at the moderate batch sizes hitherto used commercially, this is no longer sufficient to ensure the desired uniform movement of the total amount of non-fluidized particulate material when the batch size thereof is increased in an upscaled apparatus.
A more detailed explanation as to the failure of the known type of apparatuses to provide a uniform movement to large non-fluidized batches of particulate material is presented below in connection with the description of FIG. 3 of the drawing.
It has also turned out that the deficiencies of the apparatuses of known type, especially the insufficient and non-uniform powder movement, cannot be remedied by adjustment of operational features such as rotational speed of the disc.
Neither introduction of mechanical baffles into the processing chamber is sufficient to create a satisfactory particle movement in large batches. Although such means are able to provide a certain powder movement in otherwise too stationary areas, very irregular powder flow is created in the vicinity of the baffles, and in the area sheltered by the baffles particle movement becomes very unfortunate. In U.S. Pat. No. 4,724,794 it is suggested to improve the powder movement by substituting the rotary disc by plate means defining a plurality of annular slots opening at radial spacings to one another in a generally radially outward direction whereby pressurized gas blown out through said slots enhances the circulation of the charge material being treated in a radially outward direction. Preferably, the movement is also influenced by impulse blades on the rotating plate means and by stationary deflection plates, thereby imparting a treatment to the charge material of the same type as what is obtained in the above mentioned high-shear mixers.
To obtain the desired powder movement in an apparatus as the one suggested in said U.S. patent the powdery charge material will be subjected to such intense treatment that a high degree of compaction will result, for which reason the apparatus is mainly suitable for producing strong high-density particles, but less suitable for producing less compacted granules able to disintegrate into the elementary particles, as often desired when pharmaceuticals or food products are concerned.